The normal internal human aortic root conduit is provided with a sinus portion which has three sinuses (bulges) which surround the aortic valve. These sinuses are called sinuses of Valsalva and are arranged so that the cross-section of the sinus portion has a generally trefoil shape. The diameter and orifice area of the root are greater at the level of the sinus, decrease slightly at the base, but significantly decrease (by 20%) at the level of the sinotubular junction (where the sinus portion connects to the ascending portion of the aorta which supports the two iliac arteries).
The sinotubular junction or sinus ridge and the sinuses of Valsalva are known to be crucial for the normal function of the aortic valve. The sinus ridge is important in causing initial fluid flow eddies inside the sinuses of Valsalva (see Bellhouse B J: Velocity and pressure distributions in the aortic valve. J Fluid Mech 1969; 37(3): 587-600 and Bellhouse B. J.: The fluid mechanics of the aortic valve. In: Ionescu M. L., Ross D. N., Woller G. H., eds. Biological tissue heart replacement. London: Butterworth-Heinemann, 1972: 32-8). During systole, the aortic valve opens and the eddy currents created prevent the leaflets of the aortic valve from impacting on the aortic wall. Then, at the end of systole, the eddy currents inside the sinuses cause the leaflets of the aortic valve to become almost closed. Furthermore, the sinus curvature is very important in sharing stress with the leaflet. It has been demonstrated that during diastole the sinus walls move outwardly (increasing its circumferential curvature by 16%) taking up part of the load placed on the leaflet. Further it is known (see (Thubrikar M. J., Nolan S. P., Aouad J., Deck D.; Stress sharing between the sinus and leaflets of canine aortic valve. Ann Thorac Surg 1986; 42(4):434-40)) that the longitudinal length of the sinus changes very little or does not change at all during the cardiac cycle. In other words during the functioning of the aortic valve the sinus moves up and down as a whole without changing its length.
The standard surgical approach in patients with ascending aortic aneurysm or dissection involving the aortic root and associated with aortic valve disease is the replacement of the aortic valve and ascending aorta by means of a composite and valved graft onto which are reattached the two coronary arteries as originally described by Bentall and de Bono in their classical paper (Bentall H. H., De Bono A.: A technique for complete replacement of the ascending aorta, Thorax 1968; 23: 338-9). The “open” (Carrel button) method of coronary reimplantation was later recommended to decrease the tension on the coronary ostia while minimizing the risk of late false aneurysm formation. This “Carrel button” method has already reduced the incidence of pseudoaneurysm formation mainly through the reduction of the tension on the ostial anastomoses (see Svensson L. G.; Crawford E. S.; Hess K. R.; Coselli J. S.; Safi H. J.; Composite valve graft replacement of the proximal aorta: comparison of techniques in 348 patients. Ann Thorac Surg 1992, 54(3) 427-370). A modification of the standard technique was also introduced by Cabrol et al (Cabrol C, Pavie A, Gandjbakhch I. et al: Complete replacement of the ascending aorta with reimplantation of the coronary arteries. New Surgical approach, J Thorac Cardiovasc Surg 1981: 81; 309-15) for those cases of difficult presentation (low lying coronary ostia, calcified coronary ostia, tissue fibrosis in redo cases) where the coronary ostia are reattached to the aortic conduit by interposition of a small conduit.
If the aortic valve leaflets are normal, a valve-sparing aortic root remodeling procedure which keeps the natural patient valve on site is a reasonable alternative in certain individuals. David and Feindel (David T. E., Feindel C. M.: An aortic valve-sparing operation for patients with aortic incompetence and aneurysm of the ascending aorta, J Thorac Cardiovasc Surg 1992; 1 03(4): 617-21) described a surgical technique where the dilated aortic root is replaced with a tube made of DACRON fibers and the native aortic valve is integrated within the graft. This method is generally known as the “Tirone David Type I aortic valve sparing procedure”. However, the lack of sinuses in a straight tube graft was found to negatively influence proper valve function, with the consequent risk of decreasing valve longevity (Kunzelman K. S., Grande K. J., David T. E., Cochran R. P., Verrier E. D.: Aortic root and valve relationships. Impact on surgical repair J Thorac Cardiovascular Surg 1995; 109(2): 345-51).
Thus in the Tirone David Type I technique for valve sparing operations, the use of a straight tube without a sinus component raises several problems: opening and closing of the native valve is not optimal. For example, upon valve opening, the leaflets might impact on the graft and be potentially damaged. The absence or delay in eddy current formation might alter valve closure causing some regurgitation. Furthermore, the diastolic stress is borne only by the leaflet and is not shared with the sinuses causing a potential decrease in leaflet longevity.
An optimal design for root replacement should therefore incorporate sinuses and a sinotubular junction and further refinement of the technique consisted of trimming one end of the aortic tube graft to produce three separate extensions designed to replace the three sinuses. The reshaped DACRON tube was then sutured to the aortic valve remnants (see David T. E., Feindel C. M., Bos J.: Repair of the aortic valve in patients with aortic insufficiency and aortic root aneurysm. J Thorac Cardiovasc Surg 1995; 109(2):345-51) to obtain a final configuration resembling more closely the native aortic root. A similar technique was also described by Yacoub el al (Saram M. A., Yacoub M.: Remodeling of the aortic valve annulus. J Thorac Cardiovasc Surg 1993; 105(3): 435-8) several years previously.
In U.S. Pat. No. 5,139,515 it was proposed to provide an aortic graft having a lower portions provided with “bulges” apparently mimicking the sinuses of Valsalva. However no method to produce such a conduit for use in aortic surgery is described in the patent. U.S. Pat. No. 5,139,515 described a conduit having an “annular wall of a crimped material similar to that of conventional prosthesis”. No indication is actually given of how to obtain the “annually-spaced radially outward bulges” mimicking the sinuses. Moreover the drawings clearly show that the conduit, including the sinus portion, is provided along its whole length with corrugations which lie perpendicularly to the longitudinal axis of the prosethesis, and which impart longitudinal elasticity to the whole of the conduit. Upon implantation, the graft cannot expand radially outwardly, but has the potential to move and extend in the longitudinal direction of the longitudinal axis of the prosthesis.
Or as disclosed in U.S. Pat. No. 6,352,554, a conduit may comprise two distinct tubular portions having a common axis. The first upper portion is made form a standard aortic conduit and is provided with circumferentially extending corrugations successively provided along the axis of the tubular first portion. The second lower portion, or skirt portion is a tube which can be made of the same material as the first portion (that is, any suitable biocompatible material, but preferably DACRON or PTFE) but which is provided with longitudinally extending pleats or corrugations. Each of these corrugations extends in the general direction of the longitudinal axis of the prosthesis and is positioned substantially perpendicularly to the circumferential corrugations of the first portion.
The proximal end of skirt portion 14 is attached to the distal end portion of the first portion 12 so the two connected portions have essentially the same lumen and form the tubular conduit 10.
Notwithstanding the above it is still preferred to have a single conduit, that can limit leakage, and avoid the need to connect two or more tubes to form the conduit.
Therefore there is still a need for an effective prosthetic conduit to replace the aortic root while providing all the advantages of the natural sinuses of Valsalva.